1. Field of the Invention
The present invention relates to a cooling mechanism for a circuit board used in an electronic apparatus provided with a plurality of electronic components and particularly to a mechanism for cooling the respective electronic components using a cooling fan.
2. Description of the Background Art
In electronic apparatuses such as copiers, personal computers and televisions, various circuit boards each having a plurality of electronic components mounted on a board are used. Since such circuit boards generally produce heat as the electronic apparatus operate, a cooling mechanism needs to be provided so as not to cause deterioration and damage of the electronic components due to heat production. Conventionally, a cooling mechanism adopting natural heat radiation by attaching a metallic heat radiation plate to each electronic component and radiating heat into the air after transferring the heat of the electronic components to the heat radiation plates, or a forcible heat radiation by exchanging air around the circuit boards by means of a cooling fan or a combination of these heat radiations.
On the other hand, there is a possibility that electromagnetic radiation noises produced by the electronic components on the circuit board leak to the outside and influence electronic products around. As a countermeasure to prevent the leakage of the electromagnetic radiation noises has been used a method according to which the circuit boards are accommodated in a metallic casing for shutting off the electromagnetic radiation noises. In such a case, the cooling fan installed for cooling the boards is often mounted in the metallic casing. In this way, the cooling fan is often disposed at a position distanced from the boards. Therefore, there have been problems of reducing cooling efficiency due to longer distances between the cooling fan and the circuit boards and enlarging the apparatus to provide an air blowing path.
As the miniaturization and the higher density of electronic apparatuses are required, electronic components have come to be mounted on boards with a higher density. Thus, a positional relationship between the cooling fan and the heat radiation plates or the respective electronic components cannot be sufficiently rationalized, whereby the heat radiation plates cannot be always disposed on the air blowing path. Therefore, there is a problem of being unable to obtain a sufficient cooling effect.
Accordingly, there have been proposed methods for efficiently cooling circuit boards. For example, Japanese Unexamined Patent Publication No. H08-125363 discloses a construction in which a cooling unit having an electric fan is mounted on an integrated circuit and the integrated circuit and the cooling unit are covered by a shielding element. This construction is designed to efficiently cool the integrated circuit and shield electromagnetic radiation noises. Further, Japanese Unexamined Patent Publication No. 2002-182545 discloses an image forming apparatus in which an air duct for dividing air blown onto wired boards by means of a cooling fan is provided on a holder cover for holding a contact member, thereby efficiently blowing air to heat-producing portions of the wired boards. Japanese Unexamined Patent Publication No. 2000-332474 discloses a method according to which, in an electronic-component cooling construction realized by fixing a heat sink (heat radiation plate) and a fan for cooling the heat sink to a circuit board, an air blowing path from the cooling fan and a lateral fin for guiding the air from the cooling fan sideways are provided on the heat sink.
However, the method disclosed in Japanese Unexamined Patent Publication No. H08-125363 requires a space for disposing the shielding element for covering the integrated circuit and the cooling unit. The method disclosed in Japanese Unexamined Patent Publication No. 2002-182545 also requires a space for disposing the cooling fan and the shielding member since the cooling fan is provided on the frame and electromagnetic waves are shielded by covering the wired board from above and below by means of the cooling fan and the shielding member. In either case, such a space hinders the miniaturization and the space-saving of the apparatus. Japanese Unexamined Patent Publication No. 2000-332474 does not refer to the countermeasure against the electromagnetic radiation noises at all.